What is the function of From and To at the DATA section of smtp mail?
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Understanding 'From' and 'To' in SMTP DATA Section
Explore the critical roles of the 'From' and 'To' fields within the DATA section of an SMTP email, differentiating them from their envelope counterparts and understanding their impact on email delivery and display.
When sending an email via SMTP, it's common to encounter 'From' and 'To' fields in two distinct contexts: the SMTP envelope (MAIL FROM/RCPT TO commands) and the email's DATA section (message headers). While they often contain similar information, their functions are fundamentally different. This article will clarify the purpose of the 'From' and 'To' fields specifically within the DATA section, explaining how they influence how an email is displayed to the recipient and processed by mail clients.
The SMTP Envelope vs. Message Headers
Before diving into the DATA section, it's crucial to understand the distinction between the SMTP envelope and the message headers. The SMTP envelope is used by mail servers to route the email, much like a physical envelope guides a letter through the postal system. The MAIL FROM command specifies the sender for bounce messages, and RCPT TO specifies the actual recipients for delivery. These are part of the SMTP conversation itself, not the email content.
Conversely, the message headers, including the 'From' and 'To' fields, are part of the email's content (the DATA section). These headers are what the recipient's email client typically displays to the user. They provide metadata about the message, such as the sender's display name, the subject, and the date.
sequenceDiagram
participant Client
participant Sending_MTA as Sending Mail Server
participant Receiving_MTA as Receiving Mail Server
participant Recipient_Client as Recipient Mail Client
Client->>Sending_MTA: Connect
Client->>Sending_MTA: HELO/EHLO
Client->>Sending_MTA: MAIL FROM:<envelope-sender@example.com>
Client->>Sending_MTA: RCPT TO:<envelope-recipient@example.com>
Client->>Sending_MTA: DATA
Note over Client,Sending_MTA: Start of DATA section
Client->>Sending_MTA: From: "Display Name" <header-from@example.com>
Client->>Sending_MTA: To: "Recipient Name" <header-to@example.com>
Client->>Sending_MTA: Subject: Your Message
Client->>Sending_MTA: \nBody of the email message.
Client->>Sending_MTA: .
Note over Client,Sending_MTA: End of DATA section
Sending_MTA->>Receiving_MTA: SMTP Transaction (using envelope addresses)
Receiving_MTA->>Recipient_Client: Deliver message
Note over Receiving_MTA,Recipient_Client: Recipient Client displays header 'From' and 'To'SMTP Email Flow Highlighting Envelope vs. Header Usage
The 'From' Header Field
The From: header field in the DATA section specifies the author(s) of the message. This is the address that email clients typically display as the sender to the recipient. It can include a display name, which makes the sender more recognizable and user-friendly.
Its primary function is for presentation to the end-user. While it often matches the MAIL FROM envelope address, it doesn't have to. For instance, a mailing list might have a MAIL FROM address that points to the list server for bounce handling, but the From: header will show the original author of the message or the list's identity.
Mail clients and spam filters pay close attention to the From: header. Discrepancies between the From: header and the MAIL FROM (envelope sender) can sometimes trigger spam warnings, especially if not handled correctly (e.g., with proper SPF/DKIM/DMARC authentication).
From: "John Doe" <john.doe@example.com>
From: marketing@company.com
From: "Support Team" <support@company.com>
Examples of the 'From' header field
The 'To' Header Field
The To: header field in the DATA section specifies the primary recipient(s) of the message. Similar to the From: header, its main purpose is for display to the recipient. It informs the recipient who the email was primarily addressed to.
Like the From: header, the To: header does not directly control email routing. The actual delivery is determined by the RCPT TO commands in the SMTP envelope. It's possible for an email to be delivered to an address that is not listed in the To: header, for example, if the recipient was included in the Cc: (Carbon Copy) or Bcc: (Blind Carbon Copy) fields.
For Bcc: recipients, their addresses are never included in the To: or Cc: headers of the delivered message, ensuring their privacy. The To: header will only show the primary recipients visible to all others.
To: "Jane Smith" <jane.smith@example.com>
To: team@project.com
To: "Alice" <alice@example.com>, "Bob" <bob@example.com>
Examples of the 'To' header field
From: header is well-formed and represents your sender identity clearly. This improves deliverability and recipient trust. For transactional emails, it's often best to match the domain of your From: header with your MAIL FROM envelope sender.